that such is the case), then we find, in the earliest fauna of the Eocene epoch to which our investigations carry us, the two divisions of the Ungulata completely differentiated, and no trace of any common stock of both, or of five-toed predecessors to either.  With the case of the Horses before us, justifying a belief in the production of new animal forms by modification of old ones, I see no escape from the necessity of seeking for these ancestors of the Ungulata beyond the limits of the Tertiary formations.

I could as soon admit special creation, at once, as suppose that the Perissodactyles and Artiodactyles had no five-toed ancestors.  And when we consider how large a portion of the Tertiary period elapsed before Anchitherium was converted into Equus, it is difficult to escape the conclusion that a large proportion of time anterior to the Tertiary period must have been expended in converting the common stock of the Ungulata into Perissodactyles and Artiodactyles.

The same moral is inculcated by the study of every other order of Tertiary monodelphous Mammalia.  Each of these orders is represented in the Miocene epoch:  the Eocene formation, as I have already said, contains Cheiroptera, Insectivora, Rodentia, Ungulata, Carnivora, and Cetacea.  But the Cheiroptera are extreme modifications of the Insectivora, just as the Cetacea are extreme modifications of the Carnivorous type; and therefore it is to my mind incredible that monodelphous Insectivora and Carnivora should not have been abundantly developed, along with Ungulata, in the Mesozoic epoch.  But if this be the case, how much further back must we go to find the common stock of the monodelphous Mammalia?  As to the Didelphia, if we may trust the evidence which seems to be afforded by their very scanty remains, a Hypsiprymnoid form existed at the epoch of the Trias, contemporaneously with a Carnivorous form.  At the epoch of the Trias, therefore, the Marsupialia must have, already existed long enough to have become differentiated into carnivorous and herbivorous forms.  But the Monotremata are lower forms than the Didelphia, which last are intercalary between the Ornithodelphia and the Monodelphia.  To what point of the Palaeozoic epoch, then, must we, upon any rational estimate, relegate the origin of the Monotremata?

The investigation of the occurrence of the classes and of the orders of the Sauropsida in time points in exactly the same direction.  If, as there is great reason to believe, true Birds existed in the Triassic epoch, the ornithoscelidous forms by which Reptiles passed into Birds must have preceded them.  In fact there is, even at present, considerable ground for suspecting the existence of Dinosauria in the Permian formations; but, in that case, lizards must be of still earlier date.  And if the very